Added notes in gitian-signing for optionally generating SHA256SUMS

Summary:
We did this for the 0.18.0 release, so I'm recording the process here.
I marked it as optional as this is already being supersceded by signing the binaries
individually so that automatic sig verification can be applied.

Test Plan: I did this for the 0.18.0 release

Reviewers: deadalnix, schancel, Mengerian, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1664

doc/gitian-signing.md

@@ -16,5 +16,26 @@ with the siginging process and that it's unacceptable to blindly sign builds.
 
 ## Sign your gitian builds
 
+### Sign your gitian builds individually
+
 TODO: Add scripts and instructions for checking for revoked keys and signing builds.
 For now, refer to `contrib/check-keys.sh` for retrieving all signing keys.
+
+### Sign your gitian builds under a single SHA256SUMS file (optional)
+
+These steps are optional, but makes sharing the build signatures easier.
+
+1. After building all binaries using gitian, collect the hashes for the builds
+    you care to sign. Exclude any debug binaries, unsigned builds, or metadata
+    files that are built as part of the gitian process, but be sure to include
+    the source used to generate the binaries. These hashes will look something
+    like this:
+
+    8bc4becb83b532d3be841438e6145372a8bce8f37e087dbffb2aedaee985c0e4  bitcoin-abc-0.18.0-aarch64-linux-gnu.tar.gz
+    deb3d15d6ccbce4725f0e0dc892931bfdcbcfa7ccbd35846ccbde90572248bed  bitcoin-abc-0.18.0-arm-linux-gnueabihf.tar.gz
+    79a2bff6109307fd64a569270eeb1259cb6bba53ff609af4e5340d13e25e80b8  bitcoin-abc-0.18.0-i686-pc-linux-gnu.tar.gz
+    f40ba895f21270d3a038361f9b2baed68df2688eaa01ad531b4ee29ee205cb98  bitcoin-abc-0.18.0-x86_64-linux-gnu.tar.gz
+    11dc3ba7f193c70879b3fc3cc716fde56880dfebfab8bb556b7a355b2e64f09d  src/bitcoin-abc-0.18.0.tar.gz
+    b83a25ad9050e7566fc6b4f5e33a78d71a39fd7d2f15e7143a37ffd37d501a17  bitcoin-abc-0.18.0-osx64.tar.gz
+2. Save those hashes into `SHA256SUMS.0.x.0` where x is the version number.
+3. `gpg --armor --clearsign --output SHA256SUMS.0.x.0.asc --sign SHA256SUMS`